Plain English Meaning
A pattern of bright lights leading up to the runway that helps a pilot find the runway when breaking out of clouds or low visibility on an instrument approach. The exact pattern depends on the runway, and if the lights are broken, the pilot has to be able to see farther before they're allowed to land.
Full Definition
A configuration of high-intensity lights arranged in a specific pattern extending outward from the runway threshold along the extended runway centerline, designed to provide visual guidance to pilots transitioning from instrument flight to visual flight during the of an instrument approach. Different ALS configurations exist (such as , , , , , SSALF, , and others), and the installed at a runway affects the visibility a pilot may use on an instrument approach. When all or part of the is , published are increased.